Biocompatible Implants and Methods of Making and Attaching the Same

ABSTRACT

The invention provides a biocompatible silicone implant that can be securely affixed to living tissue through interaction with integral membrane proteins (integrins). A silicone article containing a laser-activated surface is utilized to make the implant. One example is an implantable prosthesis to treat blindness caused by outer retinal degenerative diseases. The device bypasses damaged photoreceptors and electrically stimulates the undamaged neurons of the retina. Electrical stimulation is achieved using a silicone microelectrode array (MEA). A safe, protein adhesive is used in attaching the MEA to the retinal surface and assist in alleviating focal pressure effects. Methods of making and attaching such implants are also provided.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a divisional of U.S. patent application Ser. No.12/041,651, filed Mar. 3, 2008, which was based on and claims thebenefit of U.S. provisional application No. 60/904,919, filed Mar. 2,2007. The entire contents of both prior applications are incorporated byreference herein.

STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT

This application was made with government support under Grant Nos.DOE-(Artificial Retina program) and NSF-EEC-0310723(BMES ERC), awardedby The United States Department Of Energy and the National ScienceFoundation, respectively. The government has certain rights in theinvention.

BACKGROUND OF THE INVENTION

Silicones are polymeric materials that have one characteristic incommon: the polymer backbone is made of an alternate succession of Siand O atoms, joined together via strong, covalent inter-atomic bonds.The Si atoms are coupled to two adjacent O atoms and two organicradicals, i.e., C—H or C—R, where R is an organic group or moiety. Thevarious silicones only differ from each other via these organicradicals, e.g. methyl (—CH3), vinyl (—HC═CH2), or other organicfunctional group. Silicones are variously referred to as “polymerizedsiloxanes,” “polysiloxanes,” and “silicone polymers.” “Silicone rubbers”are included in this definition but typically include one or moreadditives, such as fillers, plasticizers, and crosslinkers. We use theterm “silicone” in its broader sense to refer to silicone polymers,whether or not modified with one or more additional components.

Silicones are notably neutral to the environment, asserting inparticular no chemical interaction with foreign molecules. They alsoexhibit very low electrical conductivity and are fully transparent tovisible or infra-red light. They absorb light photons in the UV range,typically at and below 280 nm wavelength (i.e. at and above 4.4 eVphoton energy).

In order to allow chemical coupling of silicone materials to foreignspecies, it is necessary to “open” their structure, i.e. to modifyirreversibly their atom assembly via breaking irreversibly some of theinter-atomic bonds. Unfortunately, this may not be accessible tomechanical action. In effect, given that these materials are elastic,they may change considerably their configuration through pulling withoutbreaking bonds, this being in particular a consequence of the rotationalsymmetry of the Si—O bonds. Similarly, opening the silicone structuremay not be feasible via thermal means. Silicone does not melt, sublimateor evaporate but rather condenses and transforms in a glassy andextremely fragile network at temperatures exceeding 230° C.

Due to their chemical inertness, silicones are recognized asbiocompatible materials and are widely used in practical medicalimplants. For example, an epiretinal visual prosthesis (a microelectrodearray (MEA) imbedded into or onto a silicone substrate, or applied usingphotolithography) is a device that can be implanted on the retina andconverts images into electrical signals that stimulate the retina. Theimages are received from an external camera and transfer the visualinformation to the MEA.

Unlike cells, which attach to their extracellular environment viaintegral membrane proteins called integrins, MEAs and other medicalimplants are generally affixed to adjacent tissue using surgical tacksor adhesives, which may be actually or potentially harmful to the tissueand, therefore, may limit the actual lifetime of the implant function.For example, a method currently used to fix an epiretinal visualprosthesis in place utilizes surgical tacks secured to the retina, whichcause local pressure effects, local tissue destruction, and vascularleakage. Pressure is a crucial component of the cellular environment andcan lead to pathology if it varies beyond the normal range. Disorders ofthis relationship can lead to disease states, such as glaucoma, in whichretinal ganglion cells undergo apoptosis and necrosis.

A major obstacle faced by bioengineers has been the ability to attachproteins to biocompatible substrates, and there is a continuing need forbiocompatible materials and less destructive methods of attaching themto tissues. If silicone implants are to be fixed in place in the body, away must be found to “activate” the silicone polymers to permit them tobond more readily to one or more compounds, such as cellular orextracellular proteins.

SUMMARY OF THE INVENTION

The present invention provides a new generation of biomaterials capableof interfacing with human or other living tissue; a method of makingimplants and other articles made of such materials, and a method ofaffixing a biocompatible medical implant to living tissue. In a firstaspect of the invention, a silicone article treated with a biocompatiblecompound that facilitates bonding to living tissue comprises a siliconesubstrate having at least one activated surface formed by irradiationwith laser light at a wavelength and power sufficient to eject organicspecies from the silicone substrate, and at least one compound (e.g., anRGD peptide or protein) capable of binding to one or more integrins,coupled to the activated surface. In one embodiment, the siliconearticle is a biocompatible, implantable prosthesis comprising aprosthesis partially or completely covered with a silicone having atleast one activated surface; and at least one biocompatible compoundcapable of binding to one or more integrins, coupled to said at leastone activated surface. A specific example is a microelectrode array foran epiretinal visual prosthesis. (FIG. 1).

In a second aspect, the invention provides a method of snaking abiocompatible, implantable prosthesis, comprising the steps of providingan implantable prosthesis; partially or completely covering theimplantable prosthesis with a silicone having at least one activatedsurface; and coupling at least one biocompatible compound capable ofbinding to one or more integrins to said at least one activated surface.

In a third aspect, the invention provides a method of securing animplantable prosthesis to living tissue, comprising the steps ofproviding an implantable prosthesis partially or completely covered by asilicone having at least one activated surface, the activated surfacebeing coupled to at least one biocompatible compound capable of bindingto one or more integrins; and allowing the at least one compound tointeract with cellular membrane proteins in the tissue, thereby securingthe implantable prosthesis to the tissue.

As used herein, the term “silicone article” means a physical item thatcontains, as a major component, one or more silicone polymers. Othercomponents, such as fillers, crosslinkers, plasticizers, other polymers,etc., may also be present The article can be a self-supporting object,such as a film, protective sleeve or jacket, component of a largerassembly, etc., or a coating on another object. The term “activatedsurface” is described below in detail.

The invention offers essential advantages over current practice and maybe applied to any type of silicone-containing implant. In particular, itshows how a specific laser-activated silicone surface may be utilized instrongly fixing an implant on a living tissue without interfering with(i) that tissue, and (ii) the function of the implant.

BRIEF DESCRIPTION OF THE DRAWINGS

Various features and embodiments of the invention will become more clearwhen reference is made to the appended drawings, wherein:

FIG. 1 is a schematic illustration of an MEA implanted on a retina incommunication with spectacles containing an external camera;

FIG. 2 is a schematic illustration of the prior art tacking of an MEA tothe retina;

FIG. 3 is a prior art screenshot showing the optical coherencetomography (OTC) of a retinal fold;

FIG. 4 is a prior art photograph showing vascular leakage due to focalpressure in a rat;

FIG. 5 is a schematic drawing of polymeric SiO, created by laseractivation according to one embodiment of the invention;

FIG. 6 is a schematic drawing showing the interaction of RGD segments ofproteins with integrins, according to one embodiment of the invention;

FIG. 7 is a schematic drawing showing how proteins bind integrins via anRGD segment, according to one embodiment of the invention;

FIG. 8 is a photograph of a Contortrostatin drop on a laser-activatedsurface according to one embodiment of the invention;

FIG. 9 is a photograph illustrating how Contortrostatin adheres tosilicone debris according to one embodiment of the invention;

FIG. 10 is a photograph of a porcine retina being tom from its aluminumbase by Contortrostatin coated silicone according to one embodiment ofthe invention;

FIG. 11 is graph of the force needed to tear Contortrostatin coatedsilicone from an aluminum base; and

FIG. 12 is a graph of the force needed to tear uncoated silicone from analuminum base.

DETAILED DESCRIPTION OF THE INVENTION

In a first aspect of the invention, a silicone article treated with abiocompatible compound is provided, and comprises a silicone substratehaving at least one activated surface formed by irradiation with laserlight at a wavelength and power sufficient to eject organic species fromthe silicone substrate, and at least one compound capable of binding toone or more integrins, coupled to the activated surface. In oneembodiment, the silicone article comprises or is part of an implantablemedical device or prosthesis, as described below.

Nonlimiting examples of silicone articles include silicone films,substrates, bulk objects, and silicone coatings. The silicone may bepresent as substantially pure silicone polymers or, more typically,silicone polymers containing one or more additives to enhance thearticle's mechanical, thermal, or other physical characteristics.Nonlimiting examples of such additives include fillers, such as silicaentities (e.g., foamed, granular, fibrous, etc; optionally, the siliconepolymers are coupled to these silica entities via grafting),plasticizers, and crosslinkers, which can be admixed with thesilicone-silica compounds to ensure lateral coupling between polymericchains that are attached (i.e. grafted) to the same silica piece; etc.The whole of a silicone/silica/crosslinker assembly constitutes asilicone rubber. Varying any of the individual constituents in qualityand quantity provides a nearly infinite range of silicone rubbers thatcan be activated according to the invention.

Integrins are integral membrane proteins used by cells to attach totheir extracellular environment. Treating an activated silicone surfacewith a compound capable of binding to one or more integrins makes itpossible to attach a silicone article, such as an implant, directly totissue, without resort to surgical tacks, toxic adhesives, or otherpotentially destructive means.

One type of compound capable of binding to integrins is anarginine-glycine-aspartate (RGI) peptide, or a protein containing atleast one RGD segment (FIG. 6). Extracellular matrix (ECM) proteins canhe used to bind integrins via RGD segments at the cellular interface(FIG. 7). Nonlimiting examples include fibronectin, laminin, andcollagen. Non-ECM proteins that contain one or more RGD segments areanother example of compounds capable of binding to integrins; specificexamples include Contortrostatin (CN), a low molecular weight proteinfound in snake venom, and Vicrostatin, the monomer of CN. Both of thesetwo proteins stick to integrins on tissue previously occupied by ECMproteins. CN works well (i.e., it is sticky) as it is small and has twoRGDs per molecule. Extracellular matrix proteins (such as fibronectin,laminin and collagen) do not adhere as well to the retina as they arelarge molecules with only one RGD; however they do adhere well to theactivated silicone.

To prepare a silicone substrate having at least one activated surface,laser light of sufficient wavelength and power is directed at one ormore surfaces of a silicone article, such as the top of a silicone film,a portion of a surface of a silicone prosthesis, etc., which causeschemical bond breaking and formation of unpaired electrons, as describedbelow. This “activates” the surface of the silicone article in andaround the areas that have been irradiated, making that area morechemically reactive toward other compounds.

The use of a monochromatic, intense UV light source can, under specificconditions, allow substantially instant light absorption and drive thesilicone structure to destabilize its atom configuration. This can beachieved with a laser source working in the UV range and under a pulsedregime, such as an excimer laser.

After investigating the actual optical absorption of a given silicone orsilicone rubber, a UV light wavelength (or photon energy) is chosen thatallows the material to absorb the UV photons selectively and exclusivelyon the Si—C bond electrons. Above a given power of the light source atthat wavelength (of the order of IOOMW), all Si—C bond electrons thatare present in the silicone volume that is traversed by the laser beammay be brought to absorb these UV photons quasi-simultaneously, over avery short period of time (on the order of 1-2 ns). That absorptionproduces the quasi-simultaneous breaking of these Si—C bonds, thusseparating the corresponding organic species, e.g., organic radicalsfrom the original silicone structure. While these radicals form a gasthat disperses in the environment, the Si—0 backbones of the nowpartially decomposed polymer remain as the sole part of the siliconethat has not absorbed the UV photons. Meanwhile, each of the Si atoms inthe polymer backbones is no longer fully interlinked except to twoadjacent O atoms. This leaves two unpaired electrons per Si atom. Eachof these electrons remains coupled to a corresponding positron in theatom nucleus and occupies a so-called orbital that is attached to theatom site. After laser irradiation of the original surface, these“dangling” bond electron orbitals constitute a dense one-dimensionalnetwork along each backbone on the actual silicone surface.

That network materializes the chemical “activation” of the processedsilicone surface. In effect, and as a result of the laser-processing,the surface is no longer neutral, but is negatively charged. Eventually,an electric field is established that stems from these orbitals andtends to attract (i) positively charged species to form covalentbonding, or even (ii) neutral species that come to settle on thesilicone surface and adhere to the Si—O backbones via electrostaticforces.

The end product of the laser-processed silicone surface is partiallyablated and, therefore, engraved (i.e. recessed) down to some 10 μm ormore below the original surface plane, depending on the number ofsuper-imposed irradiations. The activated surface is, therefore,originally localized in the recessed area but is not limited to it, asexplained by the discussion.

As noted above, C—H or other organic radicals are liberated duringirradiation as free entities. The cloud of chemical species that isformed by these radicals tends to project outwards nanometer-scaleparticles (or nano-particles) of the silicone (Si—O) backbones. Thesenano-particles land on and populate the silicone surface area that isadjacent to the recessed laser-irradiated parts. thus contributing tothe formation of a laser-activated silicone surface. Over that area,they form a dense layer of active species, since they contain thoseunpaired dangling bond electrons on each Si atom as mentioned above.Eventually, these species do react to the underlying virgin siliconesurface, resulting in a strongly adherent, active cover. As a result,activation of the silicone surface is no longer restricted to therecessed laser-processed surface but extends eventually far beyond it.

This extended activation is conformal to the un-recessed, originalsilicone surface. The geometry of the conformal activated surface thatsurrounds the laser-recessed parts may be tailored through the actualgeometry and distribution of these laser-processed recessed areas. Sincethe latter may be monitored by precisely positioning and/or scanning thelaser beam onto the silicone surface, the entire conformal activatedsurface may be designed through computer-monitoring of the laserpositioning on the silicone surface.

All silicones (including silicone rubbers) are accessible to theabove-described laser-induced selective decomposition and activation.Such materials may differ by the type of organic-radicals that theycontain. However, because each radical is connected to a single Si atomby a normal Si—C bond, different organic-radicals may be identicallyseparated from their silicone backbone via identical irradiationconditions, irrespective of the individual identity of theorganic-radicals and silicone formulation.

Three types of bonds are present in every silicone: Si—O, Si—C and C—H.The weakest of these bonds is Si—C; (at 318 kJ/mol), the strongest isSi—O (at 452 kJ/mol), and C—H is intermediate in strength at 411 kJ/mol.Along with that bond hierarchy, optical absorption starts at 4.3, 5.3,and 5.5 eV, for Si—C, C—H, and Si—O bond (valence) electrons,respectively. Choosing a monochromatic beam working at 5 eV photonenergy (i.e., 248 nm wavelength) restricts exclusively opticalabsorption to electrons belonging to Si—C bonds.

Increasing the actual power of a laser beam working at 5 eV shouldtherefore allow the selective decomposition of silicone that preservesthe original Si—O backbone and produces the formation of the danglingbond electrons that materialize the activation of the material.Comparatively, such 5 eV photons are not absorbed by silica additiveparts. In contrast, they may be absorbed by crosslinker molecules,whether these are a silicone polymer or siloxane. In that case, againC—H and other organic radicals are selectively separated from thebackbone of these molecules, without affecting their inter-linkingfunction.

The preferred laser source that promotes this selective opticalabsorption to the most appropriate power is an excimer laser sourceworking at 248 nm wavelength, i.e. 5.00 eV photon energy. Its actualinstant power (i.e. beam energy/ pulse duration) may vary in the rangeof 50 to 200 MW. Alternatively, another laser source is utilized, thoughnot necessarily with the same effectiveness. For example, a pulsed,quadrupled-YAG laser beam would likely operate less efficiently.

In one embodiment, the irradiation is pulsed (pulse duration beingvariable in the range 5 to 40 ns, full width, depending onmanufacturer). Pulses are usually repeated several times along a train,at fixed time intervals. The processed material may be maintained fixedduring irradiation, and the train of pulses processes the same areauntil a specific amount of ablated (activated) matter is produced. Whilebeing irradiated (i.e. during laser-scanning), the target polymericmaterial may also be displaced in front of the laser source on an X-Ytable, moving perpendicularly to the laser beam axis. An appropriatecombination of pulse repetition rate and scan velocity would ensure therequired ablation per unit area. Material displacement iscomputer-controlled to any geometry and scan-speed velocity.

The ablated species scatter around the laser-ablated area and establishthe laser-activated silicone surface. Optionally, the extent of thescatter may either be limited to a few μm or expanded to several hundredμm, using a gas jet (e.g., an inert gas, such as He) that drifts theemitted species away from the irradiated area, and the scan geometry canbe adapted to account for that scatter. In contrast, a monochromaticbeam working at a photon energy exceeding 5.5 eV induces absorption fromall valence electrons, irrespective of the bond type from which theyoriginate. At and above an appropriate instant power level, this wouldeventually drive the full ablation of silicone with no activation of theremaining silicone surface, either of the irradiated part of it or ofthe surface area surrounding it.

Excimer lasers have been used to irradiate plastics to form metallizedplastics. See U.S. Pat. No. 5,599,592 to L. Laude, entitled “Process ForThe Metallization of Plastic Materials and Products Thereto Obtained,”the entire contents of which are hereby incorporated by reference.

FIG. 5 schematically depicts a conceptualization of a laser-activatedsilicone surface according to the invention. As shown, chemicallyreactive, dangling unpaired electrons bound to the Si—O backbone areexposed at the surface. The surface is thus “activated,” and can reactwith other compounds of interest.

In one embodiment, a silicone article having at least one activatedsurface formed by irradiation with laser light at a wavelength and powersufficient to eject organic species from the silicone article isprepared according to the method described above. In addition, thearticle further comprises one or more compounds capable of binding oradhering to one or more integrins, as described above. A convenient wayto apply the compound(s) to the activated surface is to provide it as agas or liquid, the latter being particularly suited for introducinglarge molecular structures, such as peptides and proteins that areotherwise difficult to manipulate. If these are contained in a liquidsolution, coating may be done by hand (e.g.,), disposing a drop of thesolution on the irradiated surface(s) of the silicone article.

Advantageously, coupling of the compound(s) to the silicone surface isgenerally restricted to the laser-activated areas as described above.When these structures are contained in a liquid solution, a drop of thatsolution may be disposed (e.g., manually) on the silicone surface. Onlythe parts of the surface that have been activated would retain theincoming species and ensure substantial adhesion and bonding. Onnon-activated surface areas, foreign species do not adhere to the virginsilicone surface and may, therefore, be removed by washing in water,gentle scrubbing, or tapping out without affecting those species thatare strongly fixed on the activated silicone surface. Other means ofdisposing these foreign species may be practiced depending on the typeand size of the species. For example, disposal may also be performed byevaporation in a vacuum chamber, and other physical or chemical meansmay be practiced as well without affecting the particular adhesion ofthese species to the laser-activated silicone surface alone.

The type and extremely dense distribution of the laser-generatedactivated parts of the silicone polymer (namely, the Si—O backbones) ona laser-processed silicone surface allow the surface to size, and keepat once, large molecules of varied formulation and shape. This isdemonstrated, for example, in disposing protein molecular structuresonto an activated silicone surface.

One type of silicone article that can be prepared according to theinvention is a silicone implant, i.e., an implantable medical devicemade, in whole or in part, of silicone. (In other words, silicone mayconstitute substantially the entire implant, or just a part of it, suchas an outer coating, sleeve, jacket, or other protective barrier.) Inone embodiment, the silicone implant is a silicone article treated witha biocompatible compound that facilitates bonding to living tissue, andcomprises a silicone substrate having at least one activated surfaceformed by irradiation with laser light at a wavelength and powersufficient to eject organic species from the silicone substrate, and atleast one compound capable of binding to one or more integrins, coupledto the activated surface,

Because retinal cells can bind to RGD peptides and proteins containingRGD segments, the present invention can be used to make an epiretinalvisual prosthesis—a silicone-coated microelectrode array (MEA) to beimplanted in the eye. The internal limiting membrane of the retina (theinner-most layer) contains laminin, fibronectin, collagen type I and IV,protecglyeans and vitreous fibrils.

Biocompatibility of an epiretinal positioned electrode array is animportant consideration when choosing the materials for the MEA.Additionally, the surgical techniques also play a role in the success ofthe implanted array. See Long-term Histological and ElectrophysiologicalResults of an Inactive Epiretinal Electrode Array Implantation in Dogs.Invest. Ophthalmol. Vis. Sci., vol. 40, no. 9, pp. 2073-2081, August1999 by A B. Majji, the entire contents of which are hereby incorporatedby reference.

Techniques for attaching arrays to ocular tissue using biological glues,retinal tacks, and magnets are known in the art. See Bioadhesives forIntraocular Use, Retina, vol. 20 pp. 469-477, 2000, by E. Margalit etal., the entire contents of which are hereby incorporated by reference.Fabricating silicone microelectrode arrays is also known in the art, SeeRetinal Prosthesis for the Blind, Surv. Ophthalmology, 47 (2002), pages335-356 by E. Margalit, et al., the contents of which are herebyincorporated by reference. See also, U.S. Department of Energy documentUCRL-LR-153347, entitled Microfabrication of an Implantable SiliconeMicroelectrode Array for and Epiretinal Prosthesis by M. N. Maghribidated Jun. 10, 2003; Batch-fabricated thin-film Electrodes forStimulation of the Central Auditory System, IEEE Trans. Biomed. Eng.,vol. 36, o. 7, pp. 693-704, July 1989 by D. J. Anderson, et al.;Integrated-circuit Approach to Extracellular Microelectrodes, IEEETrans. Biomed Eng., vol. BME-17, pp. 238-247, 1970 by K. D. Wise et al.;Implantable Microsystems, Polyimide-based Neuroprostheses forInterfacing Nerves, Med. Device Tech., vol. 10, no. 6, pp. 28-30, July1999, by T. Stieglitz et al.; the entire contents (of all of the priorreferences) of which are hereby incorporated by reference.

In one embodiment, implanted components can include a multi-channelelectrode array as well as bi-directional telemetry and hermeticallypackaged micro-electronics. These components can perform power recovery,management of data reception and transmission, digital processing, andanalog output of stimulus current.

In one embodiment, for a silicone implant comprising a microelectrodearray (MEA), and if an extracellular matrix (ECM) protein is selected asthe compound coupled to the activated silicone surface, it isadvantageous if the ECM protein has at least one of the followingcharacteristics: (i) an RGD (arginine-glycine-aspartate) amino acidsegment to enable it to interact with retinal integrins (see FIGS. 6 and7), (ii) disulfide bonds to allow covalent interaction with silicone(iii) enzyme-cleavable regions to facilitate removal of the MEA.

A non-limiting list of polymers useful for creating flexible,micro-electrode arrays are silicone, polyimide, polydimethylsiloxane,and parylenes, such as parylene N and C, and copolymer blends ofsilicone and non-silicone polymers. Note that non-silicones like thepolyimides and parylenes, without being combined with a silicone basedpolymer, may not have activated surfaces when subjected to the excimerlaser process, but are still useful polymers for retinal implants.

In one embodiment, the activated silicone may be used for long orshort-term medical devices such as implants and drug delivery devices,and in a number of tissues, including brain (e.g., cortex), heart,liver, and eye (e.g., retina). A non-limiting list of medical devicesincludes cardiac pacemakers, cochlear implants, deep brain stimulatorsfor Parkinson's disease, and epiretinal visual prostheses. For thesedevices, establishing good contact with the surrounding tissue isimportant and thus the attachment methods of the present invention maybe used. The use and implanting of cochlear implants is known in theart. See Cochlear Prosthetics, Ann. Rev. Neurosci., vol 13, pp. 357-371,1990, by G. E. Loeb, the entire contents of which are herebyincorporated by reference. Using implants to treat Parkinsonian tremorsis also known in the art. See High-frequency Unilateral ThalmicStimulation in the Treatment of Essential and Parkinsonian Tremor, Ann.Neural., vol. 42, no. 3, pp. 292-299, September 1997, the entirecontents of which are hereby incorporated by reference. From theforegoing discussion it can be appreciated that the invention alsoprovides a method of making a biocompatible, implantable prosthesis,comprising the steps of providing an implantable prosthesis; partiallyor completely covering the prosthesis with a silicone having at leastone activated surface; and coupling at least one biocompatible compoundcapable of binding to one or more integrins to the activated surface. Inaddition, the invention provides a method of securing an implantableprosthesis to living tissue, the method comprising the steps ofproviding an implantable prosthesis partially or completely covered by asilicone having at least one activated surface, the activated surfacebeing coupled to at least one biocompatible compound capable of bindingto one or more integrins; and allowing the at least one compound tointeract with cellular membrane proteins in the tissue, thereby securingthe implantable prosthesis to the tissue.

If it becomes necessary to remove such an implant from tissue to whichit has been attached, an enzyme such as plasmin can be used cleave RGDpeptides, thereby breaking the bond between the implant and adjacentintegrins.

Examples, Tests, and Discussion Protein Attachment to a Silicone Surface

Snake venom disintegrin (Contortrostatin) is a homodimeric protein thatcontains an RGD amino acid segment and disulfide bonds that allow theprotein to attach to activated silicone. An excimer laser was used tophysically break the molecular bonds and produce dangling free bonds onthe silicone surface. Using a pipette, the Contortrostatin was droppedonto the lased silicone surface and allowed to dry.

Preparation of Retinal Tissues

Postmortem porcine eyes were prepared by removing the vitreous humorwith a vitreous cutter (Bausch and Lomb). The posterior segment of theeye was flattened by making four cuts in four different quadrants fromthe pars plana to the equator. The eye was pinned out onto a polystyrenesurface and quadrants of the retina were delicately removed. Each pieceof retina was glued (Adhesive Systems RP 1500 USP) face up (i.e.internal limiting membrane up) to a piece of aluminum and allowed to dryfor 10 minutes. During this time the retina was kept moist with drops ofsaline.

Protein Adhesive Strength

The adherence forces between the Contortrostatin-coated silicone and theretina were measured by dynamic mechanical analysis, using a BoseElectroForce 3100. Contortrostatin-coated silicone was glued (AdhesiveSystems RP 1500 USP) to a piece of plastic and lowered onto the preparedretina. The silicone piece was raised 4 mm over 10 seconds and theadhesive forces resulting from the separation of retina and aluminumwere recorded.

Results

After the excimer laser was used to physically break molecular bonds,photos were taken of the silicone surface during the attachment process.The Contortrostatin drop can be seen absorbing into the lased areas(FIG. 8) and later extending over the silicone debris on the surface(FIG. 9). To test the adhesive strength of the protein to the silicone,a simple scotch tape test was performed. The scotch tape could not heremoved from the activated surface.

Adhesive Strength to the Retina

Dynamic mechanical analysis of Contortrostatin-coated silicone andnon-laser processed silicone is graphically presented in FIGS. 11 and12. The silicone in each case was removed from the retina at 0.4mm/second. FIG. 7 shows the adhesive three of the Contortrostatin-coatedsilicone is approximately 340 mN, at which point the retina was tornaway from the aluminum surface (See Photo, FIG. 10). FIG. 12 shows theplain (non-activated) silicone is easily detached from retina after just1OmN. The green line represents a force of 1 OmN, and the blue line is adisplacement of 4 min. over 10 seconds.

While this invention has been described in connection with reference towhat are considered exemplary embodiments, the invention is not limitedto the disclosed embodiments, dimensions, and configurations but, on thecontrary, also extends to various modifications and equivalentarrangements. The invention is limited only by the appended claims andtheir equivalents.

What is claimed is:
 1. A method of securing an implantable prosthesis toliving tissue, comprising: providing an implantable prosthesis partiallyor completely covered by a silicone having at least one activatedsurface, said activated surface being coupled to at least onebiocompatible compound capable of binding to one or more integrins; andallowing the at least one compound to interact with cellular membraneproteins in the tissue, thereby securing the implantable prosthesis tothe tissue.
 2. A method as recited in claim 1, wherein the implantableprosthesis compnses a microelectrode array.
 3. A method as recited inclaim 1, wherein the biocompatible compound capable of binding to one ormore integrins comprises an arginine-glycine-aspartate (RGD) peptide, ora protein containing at least one RGD segment.
 4. A method as recited inclaim 2, wherein the compound capable of binding to one or moreintegrins comprises an extracellular matrix (ECM) protein.
 5. A methodas recited in claim 4, wherein the ECM protein is selected from thegroup consisting of fibronectin, laminin, and collagen.
 6. A method asrecited in claim 2, wherein the compound capable of binding to one ormore integrins comprises a non-ECM protein.
 7. A method as recited inclaim 6, wherein the non-ECM protein is selected from the groupconsisting of contortrostatin and vicrostatin.
 8. A method as recited inclaim 1, wherein the living tissue is selected from tissue of the eye,liver, brain, or heart.